Biaxial compression-testing machine

ABSTRACT

The machine comprises two compression assemblies, each of which is called upon to apply a monoaxial compression load to the test piece in a direction horizontal and perpendicular to the load applied by the other assembly and coplanar therewith; each assembly includes a jack, the body of which is integral with the frame, and a piston applying pressure to a vertical plate adapted to apply a load to one surface of the test piece; an opposing vertical plate, coaxial with the first plate, is connected to the frame for applying to the opposite parallel surface of the test piece a load of modulus equal to the load applied by the first plate and in opposite direction thereto. The machine is characterized in that each compression assembly is carried, independently of the other assembly, on guide means having little friction and making it possible for the assembly to move freely in a horizontal direction, parallel with the direction of application of the load by the assembly; the guide means associated with the other assembly similarly making it possible for the latter to move in a direction parallel with the direction of application of the load by the other assembly and at right angles to the direction of movement of the first assembly.

United States Patent [191 Bascoul et al.

[ Mar. 19, 1974 [5 BIAXIAL COMPRESSION-TESTING MACHINE [75] Inventors:Alain Georges Bernard Bas coul,

Toulouse; Jean-Paul Bourdes, Albi; Jacques Moulis, RamOnville-Saintagne, all of France [73] Assignee: Agence Nationale deValerisation de la Recherche, Tour Aurore, France [22] Filed: Nov. 28,1972 [21] Appl. No.: 310,094

[30] Foreign Application Priority Data Nov 29, 1971 France 71.42699 [52]U.S. Cl 73/94, 73/l03, 73/88 [51] Int. Cl. G01n 3/08 [58] Field ofSearch 73/88 C, 88 R, 89, 90, 73/94, 103

[56] References Cited FOREIGN PATENTS OR APPLICATIONS 39,988 7/1957Poland 73/94 Primary Examiner-James J. Gill Assistant Examiner-AnthonyV. Ciarlante 57 ABSTRACT The machine comprises two compressionassemblies, each of which is called upon to apply a monoaxialcompression load to the test piece in a direction horizontal andperpendicular to the load applied by the other assembly and coplanartherewith; each assembly includes a jack, the body of which is integralwith the frame, and a piston applying pressure to a vertical plateadapted to apply a load to one surface of the test piece; an opposingvertical plate, coaxial with the first plate, is connected to the framefor applying to the opposite parallel surface of the test piece a loadof modulus equal to the load applied by the first plate and in oppositedirection thereto. The machine is character ized in that eachcompression assembly is carried, independently of the other assembly, onguide means having little friction and making it possible for theassembly to move freely in a horizontal direction, parallel with thedirection of application of the load by the assembly; the guide meansassociated with the other assembly similarly making it possible for thelatter to move in a direction parallel with the direction of applicationof the load by the other assembly and at right angles to the directionof movement of the first assembly. i

7 Claims, 7 Drawing Figures SHEET 1 OF 5 FIG. 1a

PATENIED m 1 9 I974 FlG.1b

FIG

PATENTED MR 1 9 1974 SHEET 3 0F 5 FIGS 1 BIAXIAL COMPRESSION-TESTINGMACHINE The invention relates to a biaxial compression-testing machinemaking it possible to subject a test piece in the form of a right-angledparallelepiped to a biaxial field of compression stresses.

In designing concrete structures, use is at present made of thepermissible working load of the concrete defined by breaking tests undersimple tension and compression. In the case of thick structuressubjected to multiaxial stresses, however, this safety definition isadequate, and it becomes necessary to understand the real behaviour ofconcrete subjected to such stresses. A case in point is that ofstructures subjected to biaxial stresses.

There are many types of test machines in existence which are capable ofsubjecting a concrete test piece to a biaxial field of compressionstresses. These machines may be divided into two categories: one ofthese categories covers machines, known as flexible machines, consistingof elements capable of undergoing deformation as the test piece isloaded; these machines usually have the advantage of compactness and lowcost but they cannot provide accurate measurements, since thedeformation suffered by the machines themselves, and the energy ofdeformation released upon rupture, produce by no means negligeabledistortions in the results obtained.

The other category covers machines, known as rigid machines, in whichthe elements are of a size such that they undergo only minor deformationduring loading; The major difficulty faced by the designers of machinesof this type is the necessity of applying to the surfaces of the testpiece loads which remain accurately centered in all stages ofdeformation of the test piece since, if meaningful results are to beobtained, it is desirable to avoid introducing any poorly knownparasitic stresses, so that only easily measurable biaxial compressionstresses are created within the test piece. Indeed, during thedeformation of the test piece, the directions in which the loads areapplied can be longer merged with the axes of thetest piece, sinceeccentricity of the load has two consequences, one being the appearanceof compound bending which destroys the uniformity of distribution ofnormal stresses, and the other being the birth of dissymmetricaltangential stresses on the surfaces of the test piece.

In order to eliminate these centering defects, some machines apply theload through diaphragms subjected to specific pressures, but the maximalloads applicable by these machines are very limited, and the dimensionsof the test pieces used must be carefully adapted to those of themachine. The dimensions of the test piece cannot be altered withoutcompletely modifying the machine. 7

Other machines apply the loads through jacks. One of the most advancedmachines of this type consists of a fixed frame and a mobile framesuspended within the fixed frame by means of a plurality of rods. Thefixed frame is equipped with a jack, the piston of which, integral witha plate, may exert pressure on one surface of the test piece, while anopposing plate, attached to this frame, exerts a pressure in theopposite direction on the opposite surface of the test piece. The mobileframe comprises similar elements arranged at right angles to the firstelements and adapted to exert pressure on two other surfaces of the testpiece. This type of machine ensures centering of the stresses applied tothe surfaces of the test piece as long as the deformation thereof issmall, the mobile frame being adapted to follow the deformation of thetest piece and thus to centre the directions in which the loads areapplied (in relation to the surfaces of the test piece). However, assoon as the deformation of the test piece andtherefore the movement ofthe mobile frame becomes appreciable, the restoring forces exerted bythe suspension rods depart from the vertical and introduce parasiticstresses which increase as the movement of the mobile frame and theangle between the rods and the vertical increase. Moreover, in this typeof machine, the direction of the load applied by the mobile frame is notsuitably defined, since this frame, suspended within the fixed frame,may pivot about a vertical axis which produces a poorly knowndistribution of stresses.

It is the purpose of this present invention to provide a machine adaptedto produce automatic centering of stresses at all stages of deformationof the test piece, with a view to creating a biaxial field ofcompression stresses in the test piece and to preventing the apparatusof parasitic eccentric stresses, especially tangential stresses.

Still another purpose of the invention is to provide a machine of simpleand compact design, low weight, and relatively low production cost.

To this end, a biaxial compression machine comprises two compressionassemblies, each of which is called upon to apply a monoaxial load tothe test piece in a direction known as the loading direction horizontaland perpendicular to the direction of the load applied by the otherassembly; each of the assemblies comprises a jack, the body of which isintegral with a frame, while the piston thereof applies pressure to avertical plate adapted to apply a load to one surface of the test piece;an opposing vertical plate, connected to the frame, is arrangedcoaxially with and opposite to the plate, for the purpose of applying,to the opposite parallel surface of the test piece, a load of modulusequal to the load applied by the first plate and in the oppositedirection thereto. According to the invention, each compression assemblyis carried, independently of the other assembly, on guided means ofdisplacement, in contact with which the frame of the assembly may movewith little friction; these guide means make it possible for therelevant assembly to move freely in a horizontal direction parallel withthe direction of application of the load by the said assembly. In asimilar manner, the guide means associated with the other as sembly makeit possible for the latter to move in a direction parallel with thedirection of application of the load by this assembly, and therefore atright angles to the direction of movement of the fiirst assembly.

As will be substantiated hereinafter, a machine of this kind can produce(apart from the friction forces acting upon the guide means) a biaxialfield of compression stresses, centered on the two axes, in all stagesof defor- ,mation of the test piece; for thepurpose of substantiatingthis and making it more readily understandable, one of the compressionassemblies of the machine according to the invention is illustratedschematically in FIG. 1a, in which 1 indicates the frame of theassembly, 2 and 3 indicate the body and piston of the jack fitted tothis assembly, 4 indicates the plate associated with p iston 3, and 5indicates the opposing plate attached to frame 1.

.By examining this diagram, it will be grasped intuitively that, sinceeach compression assembly is free to move in a horizontal direction (OHin the example illustrated), the axes of test piece 6 remain, while thetest piece is being deformed, constantly merged with the directions ofloading, as a result of the forces of action and reaction occurringbetween the plate, the opposing plate, and the test piece.

In order to study the behaviour of each compression assembly and toprovide strict substantiation of the characteristic mentioned above, thefollowing symbols will be used:

C the undeformable system consisting of frame 1,

jack body 2, and opposing plate 5;

D, the displacement of system C within the reference H;V and V and yrespectively the velocity and acceleration of this system;

P the undeformable system consisting of plate 4 and piston 3 of thejack;

D the displacement of this system within the same reference as before,and V, and respectively its velocity and acceleration.

In a reference l-l V associated with system C, the axis of the jackmoves from D at velocity v with an acceleration 'y; it is to beunderstood that the test piece is loaded slowly andprogressively(between 5 and 10 minutes, for example, for a movement of a few tenthsof a millimetre), and the value of -y is extremely low.

The breakdown of compression forces acting upon the compression assemblyand upon test piece 6, shown diagrammatically in FIGS. la and 1b, is asfollows:

F the force applied by opposing plate 5 to the surface of the test piecein contact therewith;

F p the force applied by plate 4 to the surface of the test piece incontact therewith;

F v the force applied by the other compression assembly (not shown) tothe two other vertical surfaces of the test piece, F being regarded as aconstant compressive force.

If we study in detail the possible movements of the test piece and thoseof systems C and P of the first compression assembly (FIG. 1a), thereare two possible a priori cases. These will be studied consecutively:

either the centre of gravity of the test piece moves along H;

or the centre of gravity of the test piece does not move in thatdirection.

In the first case, the plate and opposing plate of the secondcompression assembly exert friction forcesf KF, wherein K is thecoefficient of friction between the material of which the plate andopposing plate are made (more particularly steel) and the material ofwhich the test piece is made (concrete).

The resultant Re of the forces acting upon the test piece in thedirection of axis H may be written:

It should be noted that Re 0 in the reference H,V selected. It is easyto imagine that the location and shape of the test piece may beconsidered as the result of the sum of the two following operations:

a displacement of the supposedly incompressible test piece subjected toa force F F F 2KF;

a compression due to two equal and opposite forces of value F Thedisplacement of the centre of gravity of the test piece is equal to (Dc)/2 and its acceleration y may be written:

If m is the mass of the sample, then of course:

e m ya Moreover, the resultants R and R,, of the forces actingrespectively on systems C and P may be written:

F F 32 M'y +m,;y

Furthermore, by applying the value of Re (equation 1) and y (equation 2)to equation (3), we obtain:

Fp Fc=F=2KF=m ('yc+7 y may be removed from equation (7) and applied toequation (8), which gives the following value for -y:

It has been seen that, if the test piece is to move, force F producingthis movement must be greater than 2KF or than the limit equal to thisvalue. F may therefore be made equal to a-ZKF, wherein a is equal orsuperior to one.

Acceleration 7 may then be written:

M m 2 M Since the loading velocity varies between zero and a certainoperational value v, this value v is negative in the reference HVselected, and in this reference it is therefore usual for the sign of 'yto be negative.

In FIG. lie the curve of y is shown as a function of a; this curve is astraight line A almost parallel with the axis of 7; actually, the slopeof this straight line is equal in absolute value to:

Coefficient of friction K is of the order of 0.2, whereas mass m of thetest piece is of the order to 3 kg and never exceeds kg; therefore: p0.2 F/lO 0.02 F

The maximal compression exerted by the compression assemblies is of theorder of some tens of tons; it may therefore be assumed that the secondcompression assembly exerts a compression of more than 1 ton, whichcorresponds to a force F 10,000 Newtons. This hypothesis may be realizedby subjecting the test piece to prior monoaxial compression greater thanor equal to this value.

This produces p 200 which corresponds well, as already indicated, to astraight line almost parallel with the axis of 3 Moreover y is zero for:

Straight line A therefore has the configuration shown in FIG. 1c, andfor small values of y (betweenly maxl and ly maxl), which are the onlyones corresponding to reality (as already indicated), the value of a isless than one.

Thus the hypothesis of the displacement of the centre of gravity of thetest piece wherein a a 1 must be rejected.

Reality is therefore the second case cited above, in

which the centre of gravity of the test piece does not 7 When loadingbegins, the increase in velocity v is not strictly zero and there is aslight amount of dissymmetry in the loading. This dissymmetry disappearswhen a stable condition is reached in which v constant and y 0;according to equation (9), this gives f 0, and, under this condition,the centering of stresses F 7 and F,

(applied by the first assembly), in relation to stresses F (applied bythe second) is maintained.

Thus the previously mentioned characteristic of the invention issubstantiated. It should be noted, however, that this substantiationoverlooks the effect of frictional forces on the means for guiding theframes. These guide means preferably consist of horizontal rails withtheir axes parallel with the loading direction of the relevant assembly;the frame of the assembly will be supported by these rails throughrolling means reducing the friction to a very low value.

By way of example, the weight of a compression assembly adapted to applya maximal load of 60 tons is of the order of 1.3 tons; the frictionalforces are of the order of 5 to 6 kg. It will be seen, therefore, thatthe ratio of these forces to the load (I e/ 10,000) makes it possible toregard them as completely negligeable.

Moreover, the rails may be placed on three ball-andsocket jointsarranged in the form of a triangle, each mounted on a means foradjusting the height of the rails carried on a substantially horizontalbase. These arrangements make it possible to set the rails completelyhorizontally, so that the force of gravity has no effect whatever on themovements of the compression assemblies.

Furthermore, according to one preferred example of embodiment, the frameof each compression assembly consists of rigid horizontal columnsarranged in parallel with the loadingdirection of the assembly andconnecting together two rigid plates located in close proximity at theends of the columns, the body of the jack being attached to one of theseplates and the opposing plate to the other. According to onecharacteristic of the invention, each plate is carried on these columnson lowfriction sliding or rolling means, which allow them to move freelyalong the columns which acts as guides therefor. The movements of thisplate are therefore guided completely parallel with the loadingdirection, and if there is any defect in the alignment of the jack axis,the transverse component of the force applied is absorbed by the columnsand is not transmitted to the test piece;

Moreover, each plate is connected, with advantage, to the correspondingcolumns by means of arms of variable length; these arms, fitted withmeans of adjusting their lengths, make it possible to centre each of theplates.

In a structure of this kind, it is also possible to interpose betweeneach plate, and the end of the corresponding jack piston, a load sensorwhich is attached to the plate, to which it transmits the loads appliedby the piston, and which measures these loads directly. It is obviousthat this measurement of the forces actually applied is much moreaccurate than traditional measurements consisting of measuring, by meansof a dynamometer, the pressure of the fluid acting in the jacks.

Moreover, tests carried out on concrete test pieces have shown that therupture range of the concrete could be completely covered by using aloading constantly inferior to half the maximum. of the other loading.For tests on concrete test pieces it is therefore useless to provide twocompression assemblies of equal power, since the total power of one willnever be used. According to another characteristic of the inventioncapable of being applied to concrete-testing machines, the elements ofone of the compression assemblies are made heavy enough to apply apredetermined maximal load, for example 60 tons, while the elements ofthe other assembly are made to be able to apply a maximal load half asgreat, for example, 30 tons.

The invention having been set forth in general, an example of embodimentthereof is illustrated by way of example in the drawings attachedhereto, wherein:

FIGS. 1a, 1b and have already been dealt with;

FIG. 2 is a simplified view in perspective designed to facilitate theunderstanding of the general structure of a machine according to theinvention;

FIG. 3 is a view of the machine from below;

FIG. 4 is a front elevation of this machine along A-A;

FIG. 5 is a section along the line B-B.

The biaxial compression testing machine, illustrated by way of example,consists of two independent compression assemblies l and 2 with nocontact between them. Assembly 1 is designed to be able to apply to thetest piece a maximal load of 60 tons along one axis, while assembly 2 isdesigned to be able to apply a maximal load of 30 tons along an axisperpendicular to, and coplanar with, the first assembly.

Each assembly comprises a rigid frame consisting of four columns 3connected, in the vicinity of their ends,

by means of locking screws 4, to steel plates 5. Each plate 5 is carriedby a shoe 6 on a guide rail 7 by means of rollers 8 adapted to roll inslides 9 and 10 having lateral tracks; the fixed slide is attached torail 7, while the mobile slide is attached to the shoe. Rollers 8 areenclosed in a cage and are arranged at 45 alternately in one directionand the other, in order to provide accurate guidance for shoe 6 inrelation to the rail and to apply, under the action of the weight of therelevant compression assembly, substantially balanced lateral loads toslides 9 and 10.

Furthermore, each rail rests upon three ball-andsocket joints 11 locatedat the apices of a triangle and carried on micrometer screws 12 on abase 13. Screws 12 make it possible to adjust the height of each joint11 and therefore to locate each rail 7 in a perfectly horizontalposition.

A hydraulic jack body 14 is attached to one of plates 5 of the frame ofeach assembly, the jack body being appropriately centered by means ofacentering pin. Attached to the other plate of this frame is an opposingplate 15. Piston 16 of the aforesaid hydraulic jack carries at its end apressure ball 17 protected by a cap 18.

The four columns 3 of each assembly carry sleeves 19 with ball races andprotective bellows 25, an arm 20 being attached to each sleeve; the fourarms 20 thus guided on these columns carry a mobile element consistingessentially of a plate 21 and a load sensor 22 of conventional type,against which pressure ball 17 of the jack piston rests. Each arm 20 ismoreover fitted with a micrometer screw 23 having right and left-handthreads and a knurled wheel 24; accu rate centering of the assemblies.Furthermore, the columns of assembly 2 (which are smaller, this beingthe low-power assembly) pass between the columns of theother assembly;these columns may be chrome-plated and straightened to ensure perfectguidance.

A machine of this kind makes it possible to reduce almost to zero anyparasitic stresses which might be introduced as a result of eccentricityof the load during the deformation of the test piece; as alreadyindicated, the only possible source of such stresses in this machine isfriction on rails 7, this frictional force being of the order of 5 kg inassembly 1 and 2 kg in assembly 2. These forces, of the order of l/12,000 of the maximal load applicable by each assembly, are completelynegligeable.

The invention having now been set forth and its interest justified, anexclusive right thereto is reserved for the entire duration of thepatent, with no limitation other than that in the following claims.

We claim:

1. A biaxial compression-testing machine which makes it possible tosubject a test piece in the form of a right-angled parallelepiped to abiaxial field of compression stresses, said machine comprising twocompression assemblies, each of which is called upon to apply amonoaxial compression load to the test piece in a direction horizontaland perpendicular to the load applied by the other assembly and coplanartherewith, each of said assemblies comprising a jack having a bodyintegral with a frame and a piston to apply pressure to a vertical plateadapted to apply a load to one surface of the test piece; an opposingvertical plate coaxial with said first plate and connected to said framefor applying to the opposite parallel surface of the test piece a loadof modulus equal to the load applied by said first plate and in theopposite direction thereto, said test machine being characterized inthat each compression assembly is carried, independently of the otherassembly, on guide means having little friction, said guide means makingit possible for the assembly to move freely in a horizontal direction,parallel with the direction of application of the load by said assembly;the guide means associated with the other assembly similarly making itpossible for the latter to move in a direction parallel with thedirection of application of the load by said other assembly, and atright angles to the direction of movement of said first assembly.

2. A testing machine according to claim 1, characterized in that saidguide means associated with each assembly consist of horizontal rails,the axes of which are parallel with the loading direction of theassembly, the frame of said assembly being supported on said rails byrolling means.

3. A testing machine according to claim 2, characterized in that thesaid rails are located on three ball-joints arranged in the form of atriangle and each mounted on height adjusting means supported on asubstantially horizontal base.

4. A testing machine according to claim 1, wherein the frame of eachcompression assembly consists of rigid horizontal columns arranged inparallel with the loading direction of said assembly and jointingtogether two rigid plates located in the vicinity of each end of saidcolumns, the jack body being attached to one of said plates and theopposing plate to the other, said machine being characterized in thateach plate is carried on said columns on low-friction sliding meansenabling it to move freely in a direction parallel with said columnsthereby acting as guides therefor, the end of the jack piston beingadapted to apply pressure to the central area of said plate.

5. A testing machine according to claim 4, characterized in that theplate in each assembly is connected to said columns by means ofvariable-length arms, said arms being equipped with means for adjustingthe length thereof for centering said plate.

6. A testing machine according to claim 4, characterized in that a loadsensor, attached to the plate in each assembly, is interposed betweensaid plate and the end of the jack piston, said sensor serving totransfer to the plate the forces exerted by the piston and to measuresaid forces.

7. A testing machine according to claim 1, designed for testing concretetest pieces, said machine being characterized in that the elements ofone of the compression assemblies are of dimensions enabling it to applya maximal predetermined load to the test piece, while the elements ofthe other assembly are of dimensions enabling it to apply a maximal loadof the order of half of that applied by the first assembly.

1. A biaxial compression-testing machine which makes it possible tosubject a test piece in the form of a right-angled parallelepiped to abiaxial field of compression stresses, said machine comprising twocompression assemblies, each of which is called upon to apply amonoaxial compression load to the test piece in a direction horizontaland perpendicular to the load applied by the other assembly and coplanartherewith, each of said assemblies comprising a jack having a bodyintegral with a frame and a piston to apply pressure to a vertical plateadapted to apply a load to one surface of the test piece; an opposingvertical plate coaxial with said first plate and connected to said framefor applying to the opposite parallel surface of the test piece a loadof modulus equal to the load applied by said first plate and in theopposite direction thereto, said test machine being characterized inthat each compression assembly is carried, independently of the otherassembly, on guide means having little friction, said guide means makingit possible for the assembly to move freely in a horizontal direction,parallel with the direction of application of the load by said assembly,the guide means associated with the other assembly similarly making itpossible for the latter to move in a direction parallel with thedirection of application of the load by said other assembly, and atright angles to the direction of movement of said first assembly.
 2. Atesting machine according to claim 1, characterized in that said guidemeans associated with each assembly consist of horizontal rails, theaxes of which are parallel with the loading direction of the assembly,the frame of said assembly being supported on said rails by rollingmeans.
 3. A testing machine according to claim 2, characterized in thatthe said rails are located on three ball-joints arranged in the form ofa triangle and each mounted on height adjusting means supported on asubstantially horizontal base.
 4. A testing machine according to claim1, wherein the frame of each compression assembly consists of rigidhorizontal columns arranged in parallel with the loading direction ofsaid assembly and jointing together two rigid plates located in thevicinity of each end of said columns, the jack body being attached toone of said plates and the opposing plate to the other, said machinebeing characterized in that each plate is carried on said columns onlow-friction sliding means enabling it to move freely in a directionparallel with said columns thereby acting as guides therefor, the end ofthe jack piston being adapted to apply pressure to the central area ofsaid plate.
 5. A testing machine according to claim 4, characterized inthat the plate in each assembly is connected to said columns by means ofvariable-length arms, said arms being equipped with means for adjustingthe length thereof for centering said plate.
 6. A testing machineaccording to claim 4, characterized in that a load sensor, attached tothe plate in each assembly, is interposed between said plate and the endof the jack piston, said sensor serving to transFer to the plate theforces exerted by the piston and to measure said forces.
 7. A testingmachine according to claim 1, designed for testing concrete test pieces,said machine being characterized in that the elements of one of thecompression assemblies are of dimensions enabling it to apply a maximalpredetermined load to the test piece, while the elements of the otherassembly are of dimensions enabling it to apply a maximal load of theorder of half of that applied by the first assembly.